The present invention is directed to an apparatus for and methods of sensing evanescent events in a fluid field. More particularly, the present invention is directed to such apparatus and methods using an evanescent field based fluid sensor which utilizes non-intrusive fiber optic technology to sense hydrodynamic conditions.
There is a need for sensors which detect hydrodynamic flow conditions, as well as fluid density conditions and variations, in a manner that reflect true conditions in that the sensor structure itself does not interfere with fluid flow at the location being monitored. For example, in monitoring fluid flow conditions over an airfoil, it is advantageous from both testing and fluid control purposes to know how the fluid environment is interacting with the airfoil at a specific, but perhaps fleeting moment. This is because slight variations in fluid dynamic conditions can over even very short periods of time give rise to situations of considerable interest. This is not only an issue in aerodynamics, but is also of great interest in medical applications where the flow of blood through the circulatory system is monitored. This is because circulating blood is constantly changing in pressure, velocity and density as a myriad of physiological conditions react with the blood stream.
The ability to detect fleeting changes in fluid flow conditions is useful in many other situations, such as but not limited to, the flow of fluids in hypersensitive chemical processing plants and the flow of gases through systems such as air conditioning ducts and gas scrubbing systems. There are many situations in which maintenance of laminar fluid flow is important, such as air induction systems of internal combustion engines, wherein laminar flow of combustion air is important to maximize efficiency in order to reduce pollutants and fuel consumption.
The need for non-intrusive, i.e., small, fluid sensors is also apparent in the marine industry in which vehicles are propelled through two fluids simultaneously, i.e., air and water, which fluids are separated by a very complex interface. Maximizing the efficiencies of hydrodynamic surfaces on marine vessels requires knowledge of what occurs or is occurring at boundary layers directly adjacent to or perhaps even perhaps within skin structure defining the surfaces.
Further examples of the need to understand and thereby control fluid flow over surfaces are exemplified by the need of next-generation lighter-than-air cargo and passenger air ships and by competition to improve the effectiveness of sails on racing boats such as America""s Cup yachts.
Currently, the complexities encountered when attempting to comprehend boundary layer flow are perhaps best understood through three scalar partial differential equations that describe conservation of momentum for motion of a viscous, incompressible fluid. These complexities are frequently expressed mathematically in one complex expression, which relates fluid density, fluid velocity, fluid pressure, body force, and fluid viscosity. This equation has few mathematical solutions. Thus, a sensor which effectively monitors boundary layer conditions would be of considerable assistance in coping with, and effectively functioning within, an area of technology that has historically been extremely difficult to comprehend due to its complexity.
In view of the aforementioned considerations, a detector for sensing variations in properties of a fluid flowing in a boundary layer adjacent to the detector comprises an optical waveguide having a core covered by a cladding. The optical waveguide has a planar surface with an optical grating pattern thereon. When a laser beam is directed through the detector, a probing beam is modulated by the grating in a way which is indicative of changes in fluid properties in the boundary layer adjacent to the grating.
In accordance with a more specific aspect of the invention, the optical waveguide is an optical fiber with a D-shaped cross-section; the optical fiber having the core disposed adjacent to the planar surface with the grating formed in the cladding adjacent to the core.
In accordance with a further aspect of the invention the grating has a first portion and a second portion, and in still a further aspect of the invention, the second portion is spaced from the first portion by a selected distance.
The invention may also be expressed as directed to a system for sensing variations in flow field intensity of a fluid flowing in a boundary layer adjacent to a body exposed to the fluid. The system comprises an optical fiber on or in the body, the optical fiber having a core covered by cladding and a D-shaped cross-section. The D-shaped cross-section defines a planar surface adjacent the core. The planar surface has an optical grating thereon. A tunable laser produces a laser beam which is directed through the optical fiber. Before passing through the optical fiber, the laser beam is directed through a beam splitter which produces a fiber probing beam and a reference beam. The fiber probing beam passes through the optical fiber and interacts with the optical grating while the reference beam is directed to a first sensor so as to produce a reference output indicative of the amplitude of the reference beam. A second sensor detects the fiber probe beam after it has been modulated by the grating and produces a modulated output indicative of the amplitude of the probe beam as modulated by the grating. A comparator is connected to the first and second sensors for receiving the reference output and the modulated output so as to produce a differential signal indicative of the flow field intensity in the boundary layer adjacent to the body.
In further aspects of the invention, the tunable laser is a narrow linewidth, tunable laser which is passed through an optical chopper disposed between the laser and the beam splitter. In still further aspects of the invention, the first and second sensors are photodiodes and the optical grating comprises at least first and second grating portions.
The invention is also directed to methods for sensing variations in properties of a fluid flowing in a boundary layer adjacent to a detector, wherein the method comprises directing a beam of laser light through an optical waveguide. The optical waveguide has a core layer covered by a cladding layer defining a planar surface with an optical grating pattern thereon. Variations in an output of the beam of laser light are detected, which variations are indicative of changes in fluid pressure or on density in the boundary layer adjacent to the grating of the optical waveguide.
The method further comprises configuring the optical waveguide as an optical fiber with a D-shaped cross-section.
In a more specific aspect of the method, the optical fiber has an optical grating with first and second portions having line spacings corresponding to first and second Bragg angles, respectively. The laser beam is forward coupled by the first portion and forward and reversed coupled by the second portion to sense fluid conditions in the boundary layer so as to modulate the laser beam output and to also provide a reference beam.